Optical modulator

ABSTRACT

The present invention relates to an optical modulator allowing appropriate interaction between signal electrodes and an optical waveguide, only by matching phases at input ends of the signal electrodes. The optical modulator includes an optical waveguide with its center part branched into two at two Y-branch waveguides, forming first and second waveguide arms, a first signal electrode transmitting a first electric signal interacting with first light propagating through the first waveguide arm in a predetermined manner, a second signal electrode transmitting a second electric signal interacting with second light propagating through the second waveguide arm in a predetermined manner, and earthed electrodes, and can match the interacting timing between the first electric signal and the first light to the interacting timing between the second electric signal and the second light, at a first input end supplying the first electric signal and a second input end supplying the second electric signal.

This application is a divisional of application Ser. No. 09/966,600,filed Oct. 1, 2001, now U.S. Pat. No. 6,678,428.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical modulator and, moreparticularly, to a Mach-Zehnder type optical modulator which allows theinteraction appropriately between signal electrodes and an opticalwaveguide, only by matching phases at input ends of the signalelectrodes.

The optical modulation system includes a direct modulation whichmodulates the intensity of light by superimposing a modulation signal ona driving current of a light-emitting element and an external modulationwhich stores information in the light by providing an optical componentfor changing the phase, frequency, strength or polarization of the lightoutside the light-emitting element. In recent years, research anddevelopment on an external optical modulator, having an excellentbroad-band property and chirping characteristic, has been considerablymade, in response to the need for a high-speed modulation and longdistance transmission.

2. Description of the Related Art

As the external optical modulator, there are an electro-opticalmodulator, a magneto-optical modulator, an acousto-optic modulator, anelectric field absorption type modulator and the like. Theelectro-optical modulator uses the electro-optical effect, themagneto-optical modulator uses the magneto-optical effect, theacousto-optic modulator uses the acousto-optic effect, and the electricfield absorption type modulator uses the Franz-Keldysh effect and thequantum-confined Stark effect.

One of the examples of the electro-optical modulator will be explained.

In the electro-optical modulator, an optical waveguide, signalelectrodes and earthed electrodes are formed on a substrate having theelectro-optical effect. The center part of the optical waveguide isbranched into two between two Y-branch waveguides to form first andsecond waveguide arms, so as to structure a Mach-Zehnder interferometer.The signal electrodes are respectively formed on the two waveguide arms,and the earthed electrodes are formed on the substrate in parallel tothe signal electrodes with predetermined intervals therebetween. Lightis made incident on the electro-optical modulator to propagate throughthe optical waveguide, branched into two at a first Y-branch waveguideto propagate through the respective waveguide arms, merged into oneagain at a second Y-branch waveguide, and outputted from the opticalwaveguide. When electric signals, for example, high-frequency signalsare applied to the respective signal electrodes, refractive indexes ofthe respective waveguide arms change due to the electro-optical effect,and hence the progression speeds of first light and second light, eachof which propagates through the first and the second waveguide arms,change. By providing a predetermined phase difference between theelectric signals, the first light and the second light are multiplexedat the second Y-branch waveguide with the different phases, whereby themultiplexed light has a mode which is different from that of theincident light, for example, a high-order mode. The multiplexed lightwith the different mode cannot propagate through the optical waveguide,and hence the intensity of the light is modulated. The Mach-Zehnder typeoptical modulator (hereinafter abbreviated to the “MZ opticalmodulator”) realizes the modulation by the process of the electricsignal→the change of the refractive index→the change of the phase →thechange of the intensity. The electro-optical modulator like the above isdisclosed in, for example, Japanese Unexamined Patent ApplicationPublication No. Hei 2-196212.

The electro-optical modulator like the above which controls the phasesof the first light and the second light independently by the respectivesignal electrodes is particularly called as a Dual-Drive opticalmodulator (hereinafter abbreviated to “DD optical modulator”).

It should be mentioned that the phases of the lights to be multiplexedin the second Y-branch waveguide correspond to the relationship betweenthe phase of the electric signal and the phase of the light at aninteraction start point at which the electric signal and the light startthe interaction. Hence, in order to obtain the predetermined phasedifference between the phase of the first light and the phase of thesecond light in the second Y-branch waveguide, it is necessary to supplyelectric signals correlating to the respective signal electrodes, byadjusting the phases of the respective electric signals to thepredetermined phases. Conventionally, the phases of the respectiveelectric signals are adjusted by using a phase compensator which isprovided outside, because a reference point for the phase adjustment isnot provided in the optical modulator.

It should be noted that, in this method of using the phase compensator,there is a disadvantage that the phase compensator needs to be adjustedfor each product. Particularly, when the phase is compensated by thecable length, there is a disadvantage that the deviation is caused afterthe adjustment according to the temperature change, due to thetemperature coefficient. Moreover, the adjustment becomes more difficultas the frequency of the electric signal becomes higher, and when aplurality of the electro-optical modulators are used through the cascadeconnection, it is necessary to adjust the phases of the respectiveelectric signals to be supplied to the respective electric-opticalmodulators, which makes the adjustment more difficult.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical modulatorwhich allows phases of first light and second light to become thepredetermined phases at an interaction start point, by matching thephases at points from which respective electric signals are supplied torespective signal electrodes, without using a phase compensator.

The aforementioned object is achieved by an optical modulator comprisinga substrate having a predetermined optical effect, an optical waveguideformed on the substrate, being branched into first and second waveguidearms at a first Y-branch waveguide and thereafter merged into one againat a second Y-branch waveguide, a first signal electrode formed on thesubstrate, for transmitting a first electric signal which interacts withthe first light propagating through the first waveguide arm in apredetermined manner, a second signal electrode formed on the substrate,for transmitting a second electric signal which interacts with thesecond light propagating through the second waveguide arm in apredetermined manner, and an earthed electrode formed on the substrate,wherein, supposing that time for the first electric signal to transmitfrom a first input end d of the first signal electrode, from which thefirst electric signal is supplied, to a first interaction start point b,at which the first light and the first electric signal start theinteraction, is first progression time t(db); time for the first lightto propagate from a branching point a of the first Y-branch waveguide,at which light inputted to the optical waveguide is branched into thefirst light and the second light, to the first interaction start point bis first propagation time t(ab), time for the second electric signal totransmit from a second input end e of the second signal electrode, fromwhich the second electric signal is supplied, to a second interactionstart point c, at which the second light and the second electric signalstart the interaction, is second progression time t(ec), and time forthe second light to propagate from the branching point a to the secondinteraction start point c is second propagation time t(ac), thedifference between an absolute value of the difference between the firstprogression time and the first propagation time and an absolute value ofthe difference between the second progression time and the secondpropagation time is 0 or the integer multiple of one-fourth of a periodT of the first and the second electric signals, which can be expressedas follows:

 |t(db)−t(ab)|−|t(ec)−t(ac)|=0  (Expression 1)

or|t(db)−t(ab)|−|t(ec)−t(ac)|=nT/4  (Expression 2)wherein n is a positive/negative integer.

Further, this can be also expressed as follows. Supposing that time forthe first light to propagate from the first interaction start point b toa merging point k of the second Y-branch waveguide at which the firstlight and the second light is merged into one is third propagation timet(bk) and time for the second light to propagate from the secondinteraction start point c to the merging point k is fourth propagationtime t(ck), an absolute value of the difference between the sum of thefirst progression time t(db) and the third propagation time t(bk) andthe sum of the second progression time t(ec) and the fourth propagationtime t(ck) is 0 or the integer multiple of one-fourth of a period T ofthe first and the second electric signals, which can be expressed asfollows:|t(db)+t(bk)−(t(ec)+t(ck))|=0  (Expression 3)or|t(db)+t(bk)−(t(ec)+t(ck))|=nT/4  (Expression 4)wherein n is a positive/negative integer.

Further, the first and the second progression times t(db), t(ec) can beadjusted by the length, width, thickness, material, interval between thefirst or second electrode and the earthed electrodes, of the respectivesignal electrodes from the respective input ends d, e to the respectiveinteraction start points b, c, or the thickness of a buffer layerbetween the substrate and the electrode. Namely, the first and thesecond progression times t(db), t(ec) can be adjusted by the geometriclength and by the progression speed of the electric signal.

This kind of optical modulator satisfies the expression 1 or theexpression 2, and hence the difference between the phase of the firstelectric signal at the first input end d and the phase of the secondelectric signal at the second input end e becomes the difference betweenthe phase of the first electric signal at the first interaction startpoint b and the phase of the second electric signal at the secondinteraction start point c. For this reason, the optical modulator allowsthe first and the second lights, which are branched at the branchingpoint a, to be respectively subjected to the interaction at the firstand the second interaction start points b, c, by the difference of thephase of the first electric signal at the first input end d and thephase of the second electric signal at the second input end e.

Therefore, in order to allow the first and the second lights, which arebranched at the branching point a and propagating with the same phase,to interact with the first and the second electric signals,respectively, by the predetermined phase difference, all that is neededis to adjust the phase difference between the phase of the firstelectric signal and the phase of the second electric signal to thepredetermined phase difference at the first and the second input ends d,e. Namely, the first and the second input ends d, e are referencepositions for adjusting the phases. For this reason, the circuitstructure on a periphery of the optical modulator can be simplifiedsince the phase compensator is not necessary.

It should be mentioned that, when the electric signal is an analogsignal, its period is a time interval in which the same waveform isrepeated, and when the electric signal is a digital signal, its periodis a time interval each of which is allocated to one bit.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by identical reference numbers, in which:

FIG. 1 is a view showing the structure of an optical modulator accordingto a first embodiment;

FIGS. 2(A) and 2(B) are elliptical views of earthed electrodes of theoptical modulator according to the first embodiment;

FIGS. 3(A), 3(B), 3(C), and 3(D) are views explaining relationshipsbetween first light and second light and a first electric signal and asecond electric signal, in the optical modulator according to the firstembodiment.

FIG. 4 is a view showing the structure of an optical modulator accordingto a second embodiment;

FIG. 5 is an omitting view of earthed electrodes in the opticalmodulator according to the second embodiment;

FIGS. 6(A) and 6(B) is a partially enlarged view of the opticalmodulator according to the second embodiment;

FIG. 7 is a view showing the structure of an optical modulator accordingto a third embodiment;

FIG. 8 is a view showing another structure of the optical modulatoraccording to the third embodiment; and

FIG. 9 is a view showing still another structure of the opticalmodulator according to the third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will beexplained with reference to the drawings. Incidentally, the samenumerals are given to the same structures in the respective drawings,and explanations thereof are omitted.

Structure of a First Embodiment

The first embodiment is the embodiment of an optical modulator accordingto the present invention, and the embodiment in which first and secondprogression times t(db), t(ec) are adjusted by the geometric length.

FIG. 1 is a view showing the structure of the optical modulatoraccording to the first embodiment.

FIGS. 2(A) and 2(B) are omitting views of earthed electrodes of theoptical modulator according to the first embodiment. In FIGS. 2(A) and2(B), the earthed electrodes are omitted in order to make the numeralsgiven to respective parts of an optical waveguide and the numerals givento respective parts of signal electrodes clear. FIG. 2A is its top viewand FIG. 2B is a sectional view taken along the A-A′ line in FIG. 2A.

As in FIG. 1 and FIGS. 2(A) and 2(B), an optical modulator 10 isstructured by a substrate 11 including an optical waveguide 12, a bufferlayer 15, a first signal electrode 13, a second signal electrode 14 andearthed electrodes 16, 17, 18.

The substrate 11 is selected according to a predetermined opticaleffect, and lithium niobate as an electro-optical crystal is selected inthis embodiment. Incidentally, there are other electro-optical crystalssuch as lithium tantalate, lithium niobate-lithium tantalate solid andthe like. It should be mentioned that a Z-cut or an X-cut is preferableas its crystal orientation, in order to produce an electro-opticaleffect efficiently.

As its section shown in FIG. 2B, the optical waveguide 12 allows thesubstrate 11 to selectively disperse metal only at the area of theoptical waveguide 12, and increases a refractive index of the area ascompared with the rest of the area.

In this embodiment, the optical waveguide 12 is formed by the thermaldiffusion of titanium (Ti). The optical waveguide 12 has two Y-branchwaveguides, and its center part is branched into two to form first andsecond waveguide arms 12 d, 12 e, so as to structure a Mach-Zehnderinterferometer. Namely, in the optical waveguide 12, one opticalwaveguide 12 a is formed from an incident end of the light to abranching point a of a first Y-branch waveguide, and one opticalwaveguide 12 h is formed from a branching point k of a second Y-branchwaveguide to an output end of the light, as shown in FIG. 2A Between thebranching point a and the branching point k, a branch waveguide 12 b,the first waveguide arm 12 d and a branch waveguide 12 f form one of thebranched two, and a branch waveguide 12 c, the second waveguide arm 12 eand a branch waveguide 12 g form the other. Further, the first waveguidearm 12 d and the second waveguide arm 12 e are formed to besubstantially parallel to each other.

The buffer layer 15 is the layer for keeping the light from beingabsorbed into the first signal electrode 13, the second signal electrode14 and the earthed electrodes 16, 17, 18, and a silicon oxide film orthe like is generally used from the perspective of the chemicalstability.

The first signal electrode 13 and the second signal electrode 14 are theelectrode of a progression wave type, and a metal such as gold (Au) oraluminum (Al) is formed on the substrate 11 in the shape of stripelines, by means of the evaporation method and the like.

The first signal electrode 13 and the second signal electrode 14 areformed with the same line width and the same thickness, in other words,these are formed to have substantially the same sectional shape.

The first signal electrode 13 is structured by including respectiveparts of a phase control part 13 a, an action part 13 b and atermination part 13 c. The action part 13 b is formed on the firstwaveguide arm 12 d. Namely, the action part 13 b and the first waveguidearm 12 d are parallel to each other. The phase control part 13 a isformed to be almost orthogonal to the action part 13 b, and thetermination part 13 c is also formed to be almost orthogonal to theaction part 13 b.

It is supposed that an intersection of the phase control part 13 a andthe action part 13 b is an interaction start point b, and anintersection of the termination part 13 c and the action part 13 b is aninteraction end point f. The interaction start point b is the point atwhich first light which propagates through the first waveguide arm 12 dand a first electric signal which transmits through the first signalelectrode 13 start the interaction. The interaction has theelectro-optical effect because the substrate 11 is made ofelectro-optical crystal. The interaction end point f is the point atwhich the interaction ends. From the interaction start point b to theinteraction end point f becomes an action length in which the firstlight and the first electric signal interact with each other. Further,an input end d to which the electric signal is inputted is formed in thephase control part 13 a on the opposite side of the interaction startpoint b, and is disposed at one end side of the substrate 11. Atermination h to which a terminating resistor is connected is formed inthe termination part 13 c on the opposite side of the interaction endpoint f, and is disposed at an end side of the substrate 11 which isopposite to the input end d.

The second signal electrode 14 is structured by including respectiveparts of a phase control part 14 a, an action part 14 b and atermination part 14 c. The action part 14 b is formed on the secondwaveguide arm 12 e. A center part of the phase control part 14 a isformed in a horseshoe shape in order to obtain a later-described length,and the part other than the horseshoe-shaped part is formed to beparallel to the phase control part 13 a. The termination part 14 c isformed to be almost orthogonal to the action part 14 b.

It is supposed that an intersection of the phase control part 14 a andthe action part 14 b is an interaction start point c, and anintersection of the termination part 14 c and the action part 14 b is aninteraction end point g. The interaction start point c is the point atwhich second light which propagates through the second waveguide arm 12e and a second electric signal which transmits through the second signalelectrode 14 start the interaction.

The interaction end point g is the point at which the interaction ends.Moreover, an input end e to which the electric signal is inputted isformed in the phase control part 14 a on the opposite side of theinteraction start point c, and is disposed at the same side with theinput end d of the first signal electrode 13. Further, a termination jto which a terminating resistor is connected is formed in thetermination part 14 c on the opposite side of the interaction end pointg.

As shown in FIG. 1, the earthed electrodes 16, 17, 18 are formed on thesubstrate 11 with predetermined intervals from the first signalelectrode 13 and the second signal electrode 14, respectively.

Supposing that time for the first electric signal to transmit from thefirst input end d to the first interaction start point b is firstprogression time t(db), time for the first light to propagate from thebranching point a to the first interaction start point b is firstpropagation time t(ab), time for the second electric signal to transmitfrom the second input end e to the second interaction start point c issecond progression time t(ec), and time for the second light topropagate from the branching point a to the second interaction startpoint c is second propagation time t(ac), the horseshoe-shaped part isdesigned by determining the length of the second signal electrode fromthe second input end e to the second interaction start point c, inconsideration of an optical distance of the optical waveguide 12 fromthe branching point a to the first interaction start point b, an opticaldistance of the optical waveguide 12 from the branching point a to thesecond interaction start point c, and the length of the first signalelectrode from the first input end d to the first interaction startpoint b, so as to satisfy the following expression:|t(db)−t(ab)|−|t(ec)−t(ac)|=0Incidentally, it means that it is designed to satisfy the expression 3as well.

Supposing that a period of the first electric signal and the secondelectric signal is T and a positive/negative integer is n, it is alsopossible to design the horseshoe-shaped part by determining the lengthof the second signal electrode from the second input end e to the secondinteraction start point c, so as to satisfy the following expression:

 |t(db)−t(ab)|−|t(ec)−t(ac)|=nT/4

Incidentally, it means that it is designed to satisfy the expression 4as well.

It should be noted that, in the above cases, the distance ab from thebranching point a to the first interaction start point b, the distanceac from the branching point a to the second interaction start point c,the distance db from the first input end d to the first interactionstart point b, and the distance ec from the second input end e to thesecond interaction start point c have the following relationships:ab>ac   (Expression 5)anddb<ec   (Expression 6)(Operations and Effects of the First Embodiment)

Next, the operations and effects of the optical modulator according tothe first embodiment will be explained.

FIGS. 3(A), 3(B), 3(C, and 3(D) are views explaining relationshipsbetween the first light and the second light and the first electricsignal and the second electric signal in the optical modulator accordingto the first embodiment.

On the left side of FIGS. 3(A), 3(B), 3(C), and 3(D), the respectiveelectric signals in the respective input ends d, e are shown, and on theright side of FIGS. 3(A), 3(B), 3(C), and 3(D), the respective electricsignals in the respective interaction start points b, c are-shown. FIG.3(A) shows an example of the first electric signal in the case of a sinewave, and FIG. 3(B) shows an example of the second electric signal inthe case of the sine wave. FIG. 3(C) shows an example of the firstelectric signal in the case of NZR (Non Return to Zero), and FIG. 3(D)shows an example of the second electric signal in the case of the NZR.

Incidentally, waveforms at the incident end, the branching point a, therespective branch waveguides 12 b, 12 c and the respective interactionstart points b, c, which are shown in FIG. 2, show the waveform of thelight propagating through the optical waveguide 12 in the case of theexpression 1.

In FIGS. 2(A) and 2(B) and FIGS. 3(A), 3(B), 3(C), and 3(D), the lightas a carrier wave, such as a laser beam, is inputted from the incidentend and propagates through the optical waveguide 12 a. The lightpropagates through the optical waveguide 12 a to reach the branchingpoint a of the first Y-branch waveguide, and is distributed to the firstlight and the second light. For this reason, the first light and thesecond light have the same states of the strength, frequency, phase andpolarization.

The first light propagates through the branch waveguide 12 b and thefirst waveguide arm 12 d to reach the first interaction start point b inthe first propagation time t(ab).

Similarly, the second light propagates through the branch waveguide 12 cand the second waveguide arm 12 e to reach the second interaction startpoint c in the second propagation time t(ac).

Meanwhile, the first electric signal, which is inputted to the firstinput end d when the laser beam reaches the branching point a, transmitsthrough the phase control part 13 a to reach the first interaction startpoint b in the first progression time t(db). Similarly, the secondelectric signal, which is inputted to the second input end e when thelaser beam reaches the branching point a, transmits through the phasecontrol part 14 a to reach the second interaction start point c in thesecond progression time t(ec).

The first propagation time t(ab), the second propagation time t(ac), thefirst progression time t(db) and the second progression time t(ec) aredesigned to satisfy the expression 1 or the expression 2. Hence, thefirst electric signal and the second electric signal, which are inputtedto the respective input ends d, e at the same time, simultaneouslyinteract with the first light and the second light, which aredistributed at the branching point a, at the respective interactionstart points b, c.

Namely, as shown in FIG. 3(A) and FIG. 3(B), when the first electricsignal is inputted to the first input end d in the state of q1, thesecond electric signal is supposed to be inputted to the second inputend e in the state of r1 (a phase difference is π).

While the-first light, which is distributed at the branching point a,propagates from S the branching point a to the first interaction startpoint b, the first electric signal in the state of q1 transmits troughthe phase control section 13 a to start the interaction with the firstlight, in the state of q1. Further, while the second light, which isdistributed at the branching point a, propagates from the branchingpoint a to the second interaction start point c, the second electricsignal in the state of r1 transmits trough the phase control section 14a to start the interaction with the second light, in the state of r1.The first light and the second light are the lights with the samestates, and hence, when the first electric signal in the state of q1starts the interaction with the first light, the optical modulator 10allows the second light, having the same status with the first light, tostart the interaction with the second electric signal in the state ofr1, in the case of satisfying the expression 1.

Similarly, when the first electric signal is inputted in the state of q2and the second electric signal is simultaneously inputted in the stateof r2 to the input ends d, e, respectively, the first electric signal inthe state of q2 starts the interaction with the first light and thesecond electric signal in the state of r2 starts the interaction withthe second light which is the same with the first light before thebranch, in the respective interaction start points b, c as well. Thesame applies to the rest.

Moreover, when the first electric signal in the state of q1 starts theinteraction with the first light, the optical modulator 10 allows thesecond electric signal in the state of r1 to start the interaction withthe second light having a predetermined phase difference to the firstlight, in the case of satisfying the expression 2.

Therefore, when the first electric signal and the second electric signalare timed to each other with the predetermined phase difference at thefirst input end d and the second input end e, it is possible that theoptical modulator 10 allows the first light and the second light tostart the interaction respectively at the interaction start points b, c,at the timing.

Moreover, when the first electric signal and the second electric signalare digital signals, each of these periods is set as the time to whichone bit is allocated, as shown in FIG. 3(c) and FIG. 3(D). Thereby, whenthe first electric signal and the second electric signal are timed toeach other with the predetermined phase difference at the first inputend d and the second input end e, it is possible that the opticalmodulator 10 allows the first light and the second light to start theinteraction respectively at the interaction start points b, c, at thetiming, similarly to the above.

Thus, the respective input ends d, e are reference positions for timingthe first electric signal to the second electric signal. When the firstelectric signal and the second electric signal which are timed with thepredetermined phase difference at the input ends d, e are supplied, itis possible that the optical modulator 10 modulates the incident lightin a predetermined manner. For this reason, delicate adjustment by usinga phase compensator is not necessary in the optical modulator 10,contrary to the conventional art.

Incidentally, in the first embodiment, the length of the phase controlpart 14 a of the second signal electrode 14 is adjusted to the aforesaidlength by forming its center part in the horseshoe shape, but it is notlimited to the above. Any shapes, such as “⊃”, “>”, a shape of “W” beingrotated by 90 degrees leftward and the like, are suitable as long as thelength from the input end e to the interaction start point c is adjustedto the aforesaid length.

Next, another embodiment will be explained.

(Structure of a Second Embodiment)

The second embodiment is the embodiment of an optical modulatoraccording to the present invention, and the embodiment in which firstand second progression times t(db), t(ec) are adjusted by a progressionspeed of an electric signal.

FIG. 4 is a view showing the structure of the optical modulatoraccording to the second embodiment.

FIG. 5 is an omitting view of earthed electrodes in the opticalmodulator according to the second embodiment.

FIGS. 6(A) and 6(B) is a partially enlarged view of the opticalmodulator according to the second embodiment. The enlarged part is arectangular portion shown by the broken line in FIG. 4. FIG. 6(A) is itstop view and FIG. 6(B) is its sectional view.

As in FIG. 4 to FIGS. 6(A) and 6(B), an optical modulator 20 isstructured by a substrate 11 including an optical waveguide 12, a firstsignal electrode 13, a buffer layer 15, a second signal electrode 24 andearthed electrodes 16, 27, 28.

In the first embodiment, the sectional shape of the first signalelectrode 13 is substantially the same with that of the second signalelectrode 14, and hence the progression speeds of the first and thesecond electric signals are the same. For this reason, in the firstembodiment, the shape of the phase control section 14 a of the secondsignal electrode 14 is devised to design the length of the phase controlpart 14 a to the predetermined length so that the phase of the secondelectric signal is adjusted. Meanwhile, in the second embodiment, thewidth and the thickness of the second signal electrode 24 are adjustedwith reference to the first signal electrode 13 to delay the progressionspeed of the second electric signal so that the phase of the secondelectric signal is adjusted. Therefore, its structure is the same withthat of the optical modulator 10 according to the first embodiment,except for the shape of the second signal electrode 24 and the shapes ofthe earthed electrodes 27, 28, and hence only the differences of thestructure will be explained.

The second signal electrode 24 is structured by including respectiveparts of a phase control part 24 a, an action part 24 b and atermination part 24 c. The action part 24 b is formed on a secondwaveguide arm 12 e. In other words, the action part 24 b and the secondwaveguide arm 12 e are parallel to each other. The phase control part 24a is designed with the width and the thickness which satisfy theexpression 1 or the expression 2 and is formed to be almost orthogonalto the action part 24 b. The termination part 24 c is also formed to bealmost orthogonal to the action part 24 b.

An interaction start point c, an interaction end point g, an input end eand a termination j are as shown in FIG. 5, having the same meanings asthose of the first embodiment.

The shapes of the earthed electrodes 27, 28 are changed from those ofthe first embodiment according to the shape of the second signalelectrode 24, as shown in FIG. 4, and are formed on the substrate 11with predetermined intervals from the first signal electrode 13 and thesecond signal electrode 14, respectively.

Here, a numerical example will be explained more specifically, accordingto FIGS. 6(A) and 6(B).

As shown in FIGS. 6(A) and 6(B), the width of the first signal electrode13 from a first input end d to a first interaction start point b isabout 5 μm, and the width of the second signal electrode 24 from thesecond input end e to the second signal electrode c is about 80 μm.Incidentally, in the respective phase control parts 13 a, 24 a, thewidths of respective electrode parts for supplying the respectiveelectric signals are broadened. The thickness of the respectiveelectrodes of the first signal electrode 13, the second signal electrode24 and the earthed electrodes 16, 27, 28 is about 30 μm. The intervalsbetween the respective signal electrodes 13, 24 and the earthedelectrodes 16, 27, 28 are about 15 μm, respectively.

Further, the thickness of the buffer layer 15 is about 1.2 μm.

(Operations and Effects of the Second Embodiment)

In the second embodiment, the second progression time t(ec) is adjustedby the phase control part 24 a as described above, which is a differentmethod from that of the first embodiment, and hence its operations andeffects are the same as those of the first embodiment. Therefore,explanations of the operations and effects are omitted.

Incidentally, in the second embodiment, the first progression time t(db)and the second progression time t(ec) are adjusted by adjusting thewidth and the thickness of the phase control part 24 a. However, theprogression speed of the electric signal can be adjusted by adjustingthe width and the thickness of the electrode, the interval between thefirst or second electrode and the earthed electrode, and the thicknessof the buffer layer between the substrate and the electrode.

The progression speed increases as the width of the electrode thins, asthe electrode thickens, as the interval between the first or secondelectrode and the earthed electrode narrows, and as the buffer layerthickens.

Therefore, as shown in Table 1, the phase control part 24 a of thesecond signal electrode 24 increases the width of the electrode in thecase of adjusting by its width, decreases the thickness of the electrodein the case of adjusting by its thickness, decreases the thickness ofthe buffer layer in the case of adjusting by the buffer layer, andincreases the interval between the first or second electrode and theearthed electrode in the case of adjusting by the interval, as comparedwith the phase control part 13 a of the first signal electrode 13 a.Further, the phase control part 24 a of the second signal electrode 24can adjust the progression speed by combinations of the width and thethickness of the electrode, the buffer layer and the interval betweenthe first or second electrode and the earthed electrode.

TABLE 1 RELATIONSHIP BETWEEN PHASE CONTROL PART OF FIRST SIGNALELECTRODE AND PHASE CONTROL PART OF SECOND SIGNAL ELECTRODE INTERVALBETWEEN THE FIRST SIGNAL THICK- OR SECOND ELECTRODE NESS OF ELECTRODEAND THICK- BUFFER EARTHED WIDTH NESS LAYER ELECTRODE FIRST THIN THICKTHICK NARROW SIGNAL ELECTRODE (PHASE CONTROL PART) SECOND THICK THINTHIN BROAD SIGNAL ELECTRODE (PHASE CONTROL PART)

Next, another embodiment will be explained.

(Structure of a Third Embodiment)

The third embodiment is the embodiment of an optical modulator accordingto the present invention, and the embodiment which is applied when aninput end of an electric signal needs to be broadened.

FIG. 7 is a view showing the structure of the optical modulatoraccording to the third embodiment.

Similarly to the first embodiment, an optical modulator 10 according tothis embodiment is structured by a substrate 11 including an opticalwaveguide 12, a first signal electrode 13, a buffer layer 15, a secondsignal electrode 34 and earthed electrodes 16, 27, 28.

When a first electric signal and a second electric signal are inputtedto a first input end d and a second input end e through a connector, itmay be necessary to broaden a distance between the first input end d andthe second input end e, corresponding to the size of the connector to beused.

For example, some of the commercially available connectors need aninterval of about 4 mm.

In this case, in the first embodiment, a distance between the firstinteraction start point b and the second interaction start point cbecomes about 4 mm as well. Since the lengths of the first and thesecond interaction parts (between the first interaction start point band the first interaction end point f and between the second interactionstart point c and the second interaction end point g) need to be equalto each other in general, when the lengths of the straight parts (thefirst waveguide arm 12 d and the second waveguide arm 12 e) of thebranch waveguide 12 are set as 30 mm, the interaction lengths become 26mm and are shortened.

For this reason, the signal electrode is structured as shown in FIG. 7in order to make the interaction lengths as long as possible.

Namely, a phase control part 34 a of the second signal electrode 34 isbent twice in a crank shape at a right angle to the second signalelectrode 34 on the optical waveguide 12, so that the distance betweenthe first interaction start point b and the second interaction startpoint c can be shortened. As this result, the interaction length can belengthened by about 2 mm to become 28 mm, and the decrease of amicrowave can be prevented because the number of the angles decreases ascompared with the phase control part 14 a of the first embodiment, whichhas the horseshoe shape.

In FIG. 7, the methods as described in the first or the secondembodiments may be used concerning the specific distance to be adjustedof the phase control part 34 a.

FIG. 8 is a modified example of the second signal electrode 34 in FIG.7.

In this modified example, the phase control part 34 a is not in thecrank shape, but the phase control part 34 a of the second signalelectrode 34, which is disposed on the optical waveguide 12, may beformed in a straight line to the end part of the substrate 11 andinclined obliquely so that an angle θ between the phase control part 34a of the second signal electrode 34 and the second signal electrode 14on the optical waveguide 12 becomes an acute angle.

In the structure shown in FIG. 8, there is only one angle in the secondsignal electrode 34 due to the phase adjustment, and therefore, theattenuation of the microwave can be decreased as compared with thestructure in FIG. 7.

FIG. 9 shows a modified example of the second signal electrode 34 ofFIG. 7 and FIG. 8.

Since the phase control part 34 a of the second signal electrode 34 isallowed to have an S shape, instead of the straight line to the end partof the substrate 11, there is no angle in the second signal electrode34. Hence, the attenuation of the microwave due to the angle can bedecreased.

It should noted that, in the first, second, third embodiments, theaction parts of the signal electrodes are respectively formed on thewaveguide arms, but these are not necessarily placed on the respectivewaveguide arms.

In the case of an electro-optic optical modulator, the refractive indexof the waveguide arm is generated by an electric field which isgenerated between the signal electrodes and the earthed electrodes.Hence, the respective action parts of the signal electrodes may beformed so that the waveguide arm is disposed in the electric field.

The invention is not limited to the above embodiments and variousmodifications may be made without departing from the spirit and scope ofthe invention. Any improvement may be made in part or all of thecomponents.

1. An apparatus comprising: first and second propagation parts; abranching part branching an input light into first and second lightswhich travel through the first and second propagation parts,respectively, a first signal electrode comprising an input end receivingan electric signal, the first signal electrode transmitting the receivedelectric signal so that the received electric signal interacts with thefirst light traveling through the first propagation part, where anelectric length of the first signal electrode is a length between theinput end of the first signal electrode and a starting point at whichthe electric signal interacts with the first light in the firstpropagation part; a second signal electrode comprising an input endreceiving an electric signal, the second signal electrode transmittingthe received electric signal so that the received electric signalinteracts with the second light traveling through the second propagationpart, where an electric length of the second signal electrode is alength between the input end of the second signal electrode and astarting point at which the electric signal interacts with the secondlight in the second propagation part, wherein the electric length of thefirst signal electrode is longer than the electric length of the secondsignal electrode, and the first light reaches the starting point atwhich the electric signal interacts with the first light in the firstpropagation part later than when the second light reaches the startingpoint at which the electric signal interacts with the second light inthe second propagation part.
 2. An apparatus as in claim 1, wherein theelectric length of the first signal electrode is adjusted by configuringa bent part between the input end of the first signal electrode and thestarting point at which the electrical signal interacts with the firstlight in the first propagation part.
 3. An apparatus as in claim 1,wherein the apparatus is a Mach-Zehnder optical modulator.
 4. Anapparatus as in claim 2, wherein the apparatus is a Mach-Zehnder opticalmodulator.
 5. An apparatus comprising: an optical modulator comprising:first and second propagation parts; a branching part branching an inputlight into first and second lights which travel through the first andsecond propagation parts, respectively, a first signal electrodecomprising an input end receiving a first electric signal, the firstsignal electrode transmitting the received first electric signal so thatthe received first electric signal interacts with the first lighttraveling through the first propagation part, a first electric length ofthe first signal electrode being a length between the input end of thefirst signal electrode and a first starting point at which the firstelectric signal interacts with the first light in the first propagationpart; a second signal electrode comprising an input end receiving asecond electric signal, the second signal electrode transmitting thereceived second electric signal so that the received second electricsignal interacts with the second light traveling through the secondpropagation part, a second electric length of the second signalelectrode being a length between the input end of the second signalelectrode and a second starting point at which the second electricsignal interacts with the second light in the second propagation part,wherein the first electric length is longer than the second electriclength, and the first light reaches the first starting point later thanwhen the second light reaches the second starting point.
 6. An apparatusas in claim 5, wherein the first electric length is adjusted byconfiguring a bent part between the input end of the first signalelectrode and the first starting point.